1. Field of the Invention
The present invention relates to hearing aids. The invention, in particular, relates to methods for wind noise suppression in hearing aid systems. The invention, more specifically, relates to methods and processing units for adaptive wind noise suppression in hearing aid systems. The invention further relates to hearing aid systems having means for adaptive wind noise suppression.
In the context of the present disclosure, a hearing aid system should be understood as a system for alleviating the hearing loss of a hearing-impaired user. A hearing aid system may be monaural and comprise only one hearing aid or be binaural and comprise two hearing aids.
In the context of the present disclosure, a hearing aid should be understood as a small, microelectronic device designed to be worn behind or in a human ear of a hearing-impaired user. Prior to use, the hearing aid is adjusted by a hearing aid fitter according to a prescription. The prescription is based on a hearing test, resulting in a so-called audiogram, of the performance of the hearing-impaired user's unaided hearing. The prescription is developed to reach a setting where the hearing aid will alleviate a hearing loss by amplifying sound at frequencies in those parts of the audible frequency range where the user suffers a hearing deficit. A hearing aid comprises one or more microphones, a microelectronic circuit comprising a signal processor, and an acoustic output transducer. The signal processor is preferably a digital signal processor. The hearing aid is enclosed in a casing suitable for fitting behind or in a human ear.
In the present context wind noise is defined as the result of pressure fluctuations at the hearing aid microphones due to turbulent airflow. As opposed hereto, acoustic sounds created by winds are not considered as wind noise here, because such sounds are part of the natural environment.
2. The Prior Art
U.S. Pat. No. 7,127,076 B2 discloses a method for manufacturing an acoustical device, especially a hearing device. A device casing is provided with an acoustical/electrical input converter arrangement with an electric output. An audio signal processing unit establishes audio signal processing of the device according to individual needs and/or purpose of the device. At least one electrical/mechanical output converter is provided. A filter arrangement with adjustable high-pass characteristic has a control input for the characteristic. The following operational connections are established: between the output of the input converter arrangement and the input of the filter arrangement, between the output of the filter arrangement and the control input, between said output of the filter arrangement and the input of the processing unit and between the output of the processing unit and the input of the at least one output converter.
U.S. Pat. No. 7,127,076 B2 also discloses a method for wind noise suppression based on output signals from two microphones. In a first step the output signals are transformed into the frequency domain and applied to a spatial filter, such as a beam former. In a second step a Wiener filter is applied to the signal output from the spatial filter. In the final step the resulting spectrum is transformed back to the time domain to produce a wind noise suppressed signal.
One problem with a system based on a configuration with a Wiener filter is, that it requires an estimate of the noise spectrum. The noise spectrum is difficult to estimate and the reliability and efficiency of the system may therefore suffer, especially when the wind noise spectrum is time varying.
U.S. Pat. No. 6,882,736 B2 discloses a method for detection and subsequent suppression of wind noise based on input from several microphones. One of the measures for reducing detected wind noises is the application of a subtraction filter. Such a subtraction filter seeks to ensure that only those signal components that emanate equally from all the microphones, are further processed and fed to the earphone. Uncorrelated wind noise, which emanates from only one microphone, is suppressed.
One problem with this system is that the wind noise is not efficiently suppressed by a simple subtraction of the microphone output signals.